spi: dt-bindings: clarify CS behavior for spi-cs-high and gpio descriptors

Behavior of CS signal in combination of spi-cs-high and gpio descriptors
is not clearly defined and documented. So clarify the documentation

Cc: linus.walleij@linaro.org
Cc: linux-gpio@vger.kernel.org
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/3bed61807fff6268789e7d411412fbc5cd6ffe2a.1607507863.git.hns@goldelico.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: goldelico

Documentation/devicetree/bindings/spi/spi-controller.yaml

@@ -42,6 +42,33 @@ properties:
         cs2 : &gpio1 1 0
         cs3 : &gpio1 2 0
 
+      The second flag of a gpio descriptor can be GPIO_ACTIVE_HIGH (0)
+      or GPIO_ACTIVE_LOW(1). Legacy device trees often use 0.
+
+      There is a special rule set for combining the second flag of an
+      cs-gpio with the optional spi-cs-high flag for SPI slaves.
+
+      Each table entry defines how the CS pin is to be physically
+      driven (not considering potential gpio inversions by pinmux):
+
+      device node     | cs-gpio       | CS pin state active | Note
+      ================+===============+=====================+=====
+      spi-cs-high     | -             | H                   |
+      -               | -             | L                   |
+      spi-cs-high     | ACTIVE_HIGH   | H                   |
+      -               | ACTIVE_HIGH   | L                   | 1
+      spi-cs-high     | ACTIVE_LOW    | H                   | 2
+      -               | ACTIVE_LOW    | L                   |
+
+      Notes:
+      1) Should print a warning about polarity inversion.
+         Here it would be wise to avoid and define the gpio as
+         ACTIVE_LOW.
+      2) Should print a warning about polarity inversion
+         because ACTIVE_LOW is overridden by spi-cs-high.
+         Should be generally avoided and be replaced by
+         spi-cs-high + ACTIVE_HIGH.
+
   num-cs:
     $ref: /schemas/types.yaml#/definitions/uint32
     description: